For the production of a hollow engine valve, three members thereof, i.e., a valve head portion, a hollow shaft portion, and a shaft end sealing material, are prepared separately, and these three members are finally joined together by welding to obtain a hollow engine valve as a finished product. This technology has so far been performed, as disclosed in Patent Document 1 to be described below. It has also been performed to join a valve head portion and a hollow shaft portion, which has one end sealed, by welding, thereby obtaining a hollow engine valve as a finished product. Sodium is sealed up in or enclosed in a hollow hole of a hollow engine valve for an exhaust valve required to show high-temperature resistance, and takes charge of cooling in regions of the engine valve, where the temperature is particularly high, ranging from the valve head portion to a section of the hollow shaft portion closer to the valve head portion. As the shape of the hollow hole of the hollow engine valve, therefore, a shape having a diameter increased inside the valve head portion, as shown in FIG. 1 of Patent Document 2 to be described later, is more desirable than a simple cylindrical shape as seen in FIGS. 2 and 3 of the Patent Document 1 described later.
With the conventional hollow engine valve, especially, the hollow engine valve for exhaust which is exposed to high temperatures, it has been common practice to use a material showing excellent characteristics involving heat resistance, such as a manganese-based heat resisting steel, or a nickel-based heat resisting steel, in the valve head portion where the temperature is highest; use an ordinary steel material or an ordinary heat resisting steel material for the hollow shaft portion or the shaft end sealing material where the temperature is not so high; and weld the three members, i.e., the valve head portion, hollow shaft portion, and shaft end sealing material, or the two members, i.e., the valve head portion and the hollow shaft portion sealed at one end, thereby constructing a finished product of a hollow engine valve. Alternatively, if particularly excellent heat resistance is required, a method of using a material showing excellent characteristics involving heat resistance for the hollow shaft portion, as for the valve head portion, has been performed.
Essentially, it is ideal to produce the entire hollow engine valve from any of the above-mentioned materials showing characteristics including excellent heat resistance. However, these materials are expensive, hard, and poor in work characteristics or workability. Thus, these materials are used only for the valve head portion where the temperature is particularly high, and an ordinary steel plate is applied to the hollow shaft portion or the shaft end sealing material where the temperature is not very high. With the methods for producing hollow engine valves, therefore, numerous technological developments have been made, particularly, in regard to the method for production of the valve head portion. There are not many cases of using the material showing characteristics including excellent heat resistance for the hollow shaft portion as well. Thus, emphasis will hereinafter be placed on technologies concerned with methods for producing the valve head portion which, in the exhaust valve, needs to use the material showing characteristics including excellent heat resistance and whose forming is difficult.
In the production of the valve head portion of the hollow engine valve, the point of development concentrates on two areas. The first area is a method of producing the leading end diameter-increased section of the valve head portion, and the second area is a technology on a method of providing a hollow hole in the valve head portion. That is, the material used for the valve head portion is excellent in heat resistance, but poor in workability, as mentioned above. Thus, when the leading end diameter-increased section is to be formed, or when the hollow hole is to be provided, a high degree of difficulty in working has to be overcome in comparison with the working of an ordinary steel material. In this connection, various inventions and devices have been accomplished thus far.
In the Patent Document 1, “a hollow valve formed by rolling up a steel plate” does not touch on details of the method for production of the valve head portion, but merely describes “forging”. Since the hollow hole therein is in a cylindrical shape, it is assumed to have been formed using a punch or a drill.
In “a method for production of a Na-filled hollow engine valve” disclosed in the Patent Document 2, “hot plastic working” is described as being used to form the diameter-increased section of the valve head portion, and “hot isostatic pressing” is described as being used to form the hollow hole. It follows, therefore, that “hot forging” forms both of them. In the invention of the Patent Document 2, “a round material of a larger diameter than the diameter of the valve shaft” is described as being used as the material for the valve head portion, and the inner diameter of the hollow hole is described as being initially “larger than the inner diameter of the finished valve hollow portion”. In FIG. 1 of the Patent Document 2, the initial inner diameter of the hollow hole is further increased during the hot forging process of the valve head portion, and the hollow hole increased in diameter is depicted as being provided inside the diameter-increased section of the valve head portion. In the actual forging process, however, it is very difficult to increase the diameter of the hollow hole to a dimension equal to or larger than the inner diameter of the hollow hole initially bored, thereby forming the hollow hole into an appropriate shape. Actually, the hollow hole is not increased in diameter, unlike that depicted in FIG. 1 of the Patent Document 2.
In “a method for production of a Na-filled hollow engine valve” disclosed in Patent Document 3 to be described later, the leading end of a pipe is first forged into a spherical form, and then formed into an umbrella shape, according to the first invention and the second invention of this document. The leading end diameter-increased section of the valve head portion is formed by this method, and this method of forming is hot forging. The difference exists that the hollow pipe remains as such in the first invention, while a highly machinable material is sealed up in the hollow portion in the second invention. The third invention, on the other hand, starts with a pipe having a highly machinable material enclosed in its hollow portion, the pipe having a larger diameter than the diameter of a hollow shaft portion to be formed, and forms one end of the pipe into an umbrella shape, and the other end of the pipe into the shape of a thin pipe. The method of forming is hot forging for both ends.
As far as the formation of the hollow hole is concerned, the first invention has the hollow hole from the start, whereas the second invention and the third invention form the hollow hole by cutting and removing the highly machinable material enclosed in the hollow portion. In all of the first to third inventions, the center of the leading end diameter-increased section of the valve head portion is lacking in the material with high resistance to heat. These inventions, however, adopt a method in which the highly heat resistant material is welded to this central part in the final process, whereby the entire outside of the leading end diameter-increased section is formed as comprising the highly heat resistant material.
In “a method for producing a Na-filled hollow engine valve” disclosed in Patent Document 4 to be described below, the valve head portion is of a conglomerate form (first invention) or in the form of a round bar (second invention), and is characterized by having a shallow hole in an end part thereof joined to the hollow shaft portion. That is, the highly heat resistant material constituting the valve head portion has poor machinability. Thus, only the shallow hole is bored, and the hollow shaft portion is welded to the bored part to form a continuous hollow hole, whereafter the valve head portion is formed into an umbrella shape. The steps of forming the valve head portion are performed by hot forging. Drawbacks characteristic of this method are as follows: The hollow hole bored in the valve head portion is the shallow hole. In the hollow engine valve as the finished product, therefore, the site of welding between the valve head portion and the hollow shaft portion is in the close vicinity of the diameter-increased section of the valve head portion, and is exposed to considerably high temperatures during the operation of the engine. This arouses concern about strength.